1. Field of the Invention
The present invention relates to an electrically heated catalytic converter having a laminated assembly of thin metal sheets.
2. Description of the Related Art
An exhaust gas purification device that utilizes a three-way reducing and oxidizing catalytic converter disposed on the exhaust passage of an internal combustion engine is commonly used. Generally, the catalyst used in such converters is able to purify the pollutants in the exhaust gas only when the temperature of the catalyst exceeds a certain temperature, i.e., the catalysts in the converter do not work when the temperature of the catalyst is below an activating temperature.
Usually, once the engine starts, the catalyst in the converter is gradually heated by the exhaust gas of the engine and reaches the activating temperature. However, when the temperature of the engine is low, such as during a cold start, it takes a substantial amount of time to heat up the catalyst to the activating temperature, since the heat of the exhaust gas is dissipated to the cold wall of the exhaust passage before reaching the converter. Therefore, during a cold start of the engine, the exhaust gas from the engine is not sufficiently purified since the temperature of the catalyst is lower than the activating temperature.
To solve this problem, electrically heated catalytic converters are used to shorten the time required for the catalyst to reach the activating temperature. Usually, electrically heated catalytic converters have metal substrates, and heat up catalysts during engine start by feeding an electric current through the metal substrates, i.e., by using the metal substrates as electric heaters.
An electrically heated catalytic converter of this type, for example, is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-179939. The electrically heated catalytic converter disclosed in JPP '939 has a substrate formed as a scroll-like cylindrical laminated assembly of thin metal sheets.
More specifically, the substrate in JPP '939 comprises a thin plain metal sheet and a thin corrugated metal sheet both provided with insulating coatings on the surfaces. The plain metal sheet and the corrugated metal sheet are placed one upon another and wound around a common center electrode so that the plain metal sheet and the corrugated metal sheet form a scroll-like cylinder of laminated metal sheets. The outer surface of the scroll-like laminated assembly is connected to an outer electrode. In the layers near the center electrode and in the layers near the outer electrode of the laminated assembly, the crests and bottoms of the corrugated metal sheet are soldered to the plain metal sheet in such a manner that electric currents can flow through the soldered parts. On the other hand, the corrugated metal sheet and the plain metal sheet are not soldered in the intermediate layers between these soldered layers, therefore, the plain metal sheet and the corrugated metal sheet are electrically isolated by the insulating coatings. After the scroll-like laminated assembly of the metal sheets is formed, a three-way reducing and oxidizing catalyst of known type is attached to the surfaces of the thin metal sheets in each layer of the laminated assembly.
When an electric voltage is imposed between the electrodes, electric current flows in a radial direction through the regions near the center electrode and the outer electrode through the soldered parts between the plain and corrugated metal sheets. In the intermediate region, the electric current flows through the metal sheets, since the plain metal sheet and the corrugated metal sheet are electrically isolated by insulating coatings.
This means that in the regions near the outer electrode and the center electrode, radial electric paths having shorter lengths and larger cross sections are formed. On the other hand a spiral electric path through the metal sheets which has a longer path length and smaller cross section is formed in the intermediate region.
Therefore, when electricity is fed to the laminated assembly, the intermediate region, which has a larger resistance than the region near the center electrode and the outer electrode, generates a large amount of heat and reaches high temperatures.
When the intermediate region of the substrate reaches the activating temperature (for example, 300.degree. to 400.degree. C.), an oxidation reaction of unburned HC and CO components in the exhaust gas starts, and once the reaction starts, the entire substrate is heated up rapidly by the heat generated by the oxidation reaction.
However, in the electrically heated catalytic converter in JPP '939, electric currents flow uniformly through the metal sheets in the intermediate region. This means that, when electricity is fed to the converter, the entire volume of the cylinder formed by the metal sheets in the intermediate region of the laminated assembly is uniformly heated by the electric current. Since the volume of the metal sheets in the intermediate region is relatively large, the total heat mass of the metal sheets in the intermediate region is also relatively large. Therefore, a relatively long time is required to heat up the metal sheets in this region to the activating temperature of the catalyst.
Though it is possible to heat up the entire volume of the metal sheets by feeding a larger electric current, it is not practical to feed large electric current to the converter during a starting operation of the engine since it increases the load on the battery and the alternator of the engine and may lead to a failure to start the engine or excessive battery wear.